1. Field of the Invention
This invention relates to a master information carrier carrying thereon information to be transferred to a slave medium.
2. Description of the Related Art
In the magnetic transfer, the magnetization pattern representing information (e.g., servo information) carried by a master information carrier is magnetically transferred from the master information carrier to a slave medium by applying a transfer magnetic field to the slave medium and the master information in close contact with each other. (See, for instance, Japanese Unexamined Patent Publication No. 10(1998)-269568)
The master information carrier employed in the magnetic transfer comprises a base sheet of, for instance, silicon or glass and an irregularity pattern which is formed on the base sheet by a magnetic material through photo-fabrication, sputtering or etching and represents the information to be transferred.
It has been proposed to form a master information carrier by photolithography which has been employed in the field of semiconductor or a stamper method which has been employed for producing an optical disc stamper.
It is important in the magnetic transfer that the signal transferred to the slave medium by the magnetic transfer can be accurately read by a recording and reproducing drive in which the slave medium is loaded.
From this viewpoint, it is necessary to enhance the transferred signal positioning accuracy by accurately positioning the slave medium and the master information carrier with respect to each other and that positioning accuracy of the transferred signal is such as to ensure that the transferred signal is positioned in an area where the drive can read the signal even if the master information carrier and/or the slave medium is deformed by the temperature change upon magnetic transfer.
The slave medium is generally a flexible disc or a hard disc. The base sheet of the flexible disc is generally of plastic film such as of PET, PEN or aramid whereas the base sheet of the hard disc is of a hard sheet such as of glass or aluminum. The coefficient of thermal expansion of the base sheet of the slave medium is as required by the drive.
The dimensions of the master information carrier change with the difference between the environmental temperature when the irregularity pattern is formed (mastering) and the environmental temperature when the signal is transferred to the slave medium, which changes the position of the signal transferred to the slave medium. In order to fix the position of the signal transferred to the slave medium, it is necessary to control the environmental temperature during magnetic transfer. However, to strictly control the temperature during the steps including the magnetic transfer is difficult and adds to the cost.
That is, resist is coated on a silicon sheet, the silicon sheet coated with the resist is imagewise exposed to an electron beam, and then the silicon sheet is developed and washed, thereby forming an original master bearing thereon a pattern representing information to be transferred to slave media (mastering step). Thereafter, the original master is plated with Ni or the like, whereby a base sheet bearing thereon a duplicate of the pattern on the original master is formed. Thereafter, the base sheet is stamped in a desired size and a magnetic layer is formed on the surface of the pattern, thereby obtaining a master information carrier.
The thickness of the resist layer and distribution of the thickness of the resist layer in the mastering step correspond to the depth of the irregularity pattern on the final master information carrier and the position of the resist exposed to the electron beam determines the regular disc position in which the slave medium is to be positioned. Accordingly, the thickness of the resist layer and the position of the resist exposed to the electron beam must be accurately controlled. For this purpose, it is necessary to use a temperature control system which controls the environmental temperature within 25° C.±0.1° C., which result in high initial cost.
Further, in the magnetic transfer step where a transfer magnetic field is applied to the master information carrier and the slave medium in close contact with each other, the temperature inside the magnetic transfer apparatus tends to rise due to repeated continuous magnetic transfer. This is caused by heat generated by various systems of the magnetic transfer apparatus itself and/or heat generated by repeatedly bringing the master information carrier and the slave medium into close contact with each other. When the temperature inside the magnetic transfer apparatus rises, also the temperature of the master information carrier held by the master holder of the magnetic transfer apparatus rises.
If the temperature change can be held within ±0.1° C. during steps including the magnetic transfer step, there arises no problem. However, in order to realize such a strict temperature control, high initial cost is required. Practically, the temperature change can reach ±2.5° C. a day, and about ±5° C. a year, and it is required that the thickness of the resist layer and the position of the resist exposed to the electron beam can be accurately controlled in spite of a temperature change to such an extent.
The slave medium, e.g., a hard disc, flows in sequence along a line including a varnishing step, a magnetic transfer step and the like after the magnetic layer is formed on the base sheet by sputtering or the like. While conveyed along the line, the temperature of the slave medium becomes substantially equal to that of the master information carrier immediately before reaching the magnetic transfer step though depending upon the manufacturing speed and/or the length of the line.
However to strictly control the temperature by accurately controlling the temperature of the overall line including the magnetic transfer step and/or suppressing the systems from generating heat is difficult to put into practice. Accordingly, when, for instance, the environmental temperature during the magnetic transfer step is 30° C., the temperature difference from the mastering step becomes as large as 5° C. and the master information carrier becomes different in dimensions due to thermal expansion from the original master produced at 25° C.
Also the base sheet of the slave medium (for instance, of aluminum or glass in the case of a hard disc, and PET film or the line in the case of a flexible disc) thermally expands during the magnetic transfer step, and when the signal is transferred in this state, the position of the transferred signal on the slave medium is shifted from the correct position where the transferred signal is to be positioned due to the difference between the coefficients of thermal expansion.